In the manufacture of integrated circuits, features are applied onto semiconductor wafers. A plurality of process steps are required for this, and various defects can occur on the wafer in that context. Defects can form in the features and substrate materials, or particles can settle onto the wafer surface. In order to detect these defects, the wafers are often examined macroscopically or even microscopically after each process step. These inspections are performed, in the context of the usually automated systems, by means of a camera that acquires images of the regions to be examined on the wafer. The images are evaluated by means of image processing in order to detect and classify defects.
In addition, features applied onto the wafers can also be measured. In this context, the feature spacings and feature widths are determined in order to ascertain deviations from target values. An offset of features with respect to features resulting from the previous process can also be identified. Such defects can once again be ascertained by way of an image processing system.
In the case of layers that are applied onto the wafer, it is likewise possible to identify, by way of the image processing system, color changes that are attributable to coating defects such as inhomogeneous layer thicknesses or regions where layers are absent.
The wafers to be inspected are usually placed on a scanning stage. The scanning stage is displaced beneath a camera or also beneath a macroscope or microscope fitted with a camera. Depending on the type of manufacturing process, regions on the wafer intended for subsequent inspection are selected and moved to with the scanning stage, and images thereof are acquired with the camera.
Because the features being manufactured are becoming increasingly small and more susceptible to defects, and as a result of higher manufacturing costs for larger-diameter wafers, 100% inspection of the wafer after the respective process steps is more often being demanded. This naturally requires a greater expenditure of time than for random inspection of a few individual regions on the wafer surface. Nevertheless, a greater throughput of wafers for inspection must also be guaranteed.
For these reasons, the images of the wafer surface are acquired “on the fly.” This normally involves moving a scanning stage, on which the wafer is placed, beneath a stationary camera. The scanning stage does not stop for an image acquisition but instead displaces the wafer through beneath the camera, generally at a constant speed. The camera acquires the images using a correspondingly short exposure time. This is often achieved by illuminating the region of the wafer located in the camera's image field with a short, high-intensity flash of light. The camera's electronic chip is exposed during this short time interval, and acquires an image of that region of the wafer.
When scanning a wafer, it is usual to orient the camera's rectangular image field so that the longer side of the image field rectangle is parallel to the scan lines. The longer side of the rectangle thus points in the direction of motion of the scanning stage.
The scanning stage is furthermore scanned in a rectangular, meander-shaped fashion in order to shorten the throughput time for wafer inspection. The displacement track thus contains, at the changeover to a scanning line to be scanned next, portions at right angles to one another.
The scanning stage is displaced over a predefined path in each scan line. The path is longer than the wafer diameter so that the wafer can be completely covered. At the end of each scan line the scanning stage is stopped and then moved to the adjacent scan line in the direction perpendicular to the scan line, and the scanning operation is then continued in the opposite direction in the new scan line. The displacement paths at the scan line changeover are thus at right angles to one another, and the displacement track is rectangular in shape. This process repeats at each scan line changeover.